Septicemia is a leading cause of morbidity and mortality among hospitalized patients. Despite the use of potent antibiotics and the newly produced monoclonal antibody against LPS, the mortality remains high. In the current studies we have isolated and characterized a number of proteins with agglutination, adhesion and antibacterial activities from the serum and the amebocytes of horseshoe crabs. The purification procedures using ion-exchange and affinity column chromatography resulted in a 55 kDa protein, limunectin, with the ability to bind to Limulus blood cells, to both gram positive and gram negative bacteria and to extracellular matrix. We have also identified a 12 kDa protein with unusual dual functional properties; it binds to endotoxin and also contains protease inhibitory activities. Coagulogen, the substrate for the clot formation, also has agglutinating activities once it is subjected to a limited proteolysis. We are in the process of studying the nature and specificity of such interactions and exploring the lectin properties of these and other components isolated from Limulus blood. Enzymes involved in the clotting cascade are also being investigated.